1. Field of the Invention
The present invention generally relates to apparatus for growing single crystals from a melt. In particular, the present invention relates to an apparatus for growing single crystals from a melt having a multilayer crucible which can be reshaped when deformed by the growth process.
2. Description of the Background Art
Large single crystals are used in a variety of applications, in particular as scintillation materials in nuclear imaging applications, such as positron emission tomography (PET), and Single Photon Emission Computed Tomography (SPECT). Some of the materials used to form these crystals include sodium chloride, potassium chloride, potassium bromide, lithium fluoride, sodium iodide, cesium iodide, etc. Additionally, single crystals are used for semiconductor applications and produced from, for example, germanium, silicon, solid solutions of tin and lead tellurides.
One of the more widely used methods of forming large single crystals includes pulling a crystal from a melt contained in a crucible using an initial seed crystal. The apparatus used for such a process involves a sealed chamber with cooled walls and a crucible accommodated in a lower portion thereof. Generally, the central axis of the chamber will be vertically aligned with the central axis of the crucible. Heaters are positioned proximate to the crucible and provided with insulating material. The apparatus also has a vertical rod extending from the upper portion of the chamber with its central axis also aligned with the central axis of the crucible. At the lower end of the rod is a holder for a crystal seed. Drivers are provided to impart torque to the rod so that it can rotate about its axis.
Generally, for operation of the apparatus, the crucible is heated to melt the contents therein and a rod is lowered until the crystal seed comes into contact with the melt material. The seed crystal will partially melt away and the temperature subsequently lowered to terminate melting and begin freezing of the melt onto the seed. The rotating rod is slowly pulled and the temperature is controlled to affect the growth of the crystal and prevent further growth when the crystal has reached the desired dimensions.
A representative single crystal pull apparatus according to the prior art is illustrated in FIG. 1. As can be seen therein, the apparatus has a lower chamber 1 and an upper chamber 2. Extending from the upper chamber 2 and into the lower chamber 1 is a vertical rod 3 whose rotational axis is aligned with the central axis of the upper chamber 2. At the lower end of the vertical rod 3 is a holder 4 for holding the crystal seed 5. The crystal seed 5 is used for initiating the formation of a new single crystal from the melt material in lower chamber 1. A motor drive is used to raise and lower the vertical rod 3 as well as impart axial rotation thereto. Arranged in the lower chamber 1 is the crucible 6 whose central rotation axis is also aligned with the rotational axis of vertical rod 3. The crucible 6 is made up of an inner crucible 7 as well as an outer container 8. Outer container 8 forms an annular well around the inner crucible 7 and is spaced above the level of any melt contained in the inner crucible 7. Apertures 9 are formed in the bottom of the wall of outer container 8 to allow entry of melt material from the outer container 8 to the inner crucible 7. Mounted on the edge of the outer container 8 is an outwardly projecting flange 10 which rests on a support ring 11. The support ring 11 is carried by a cylindrical stand 12 which is mounted to a gear 13.
A feeding tube 14 is also arranged to supply the raw material to be melted, usually a comminuted powder. The powder is charged to the outer container 8 surrounding the inner crucible, which powder is thereafter melted and passes through the apertures 9 into the inner crucible 7. A driver is also generally used to charge the raw material to the feeding tube 14.
Furthermore, side heaters 15 can be positioned around the sides of the crucible 7 and lower heaters 16 can be positioned below the crucible 7. The side heaters 15 and lower heaters 16 can be controlled independently of one another so the temperature can be more easily managed and controlled. The side heaters 15 and lower heaters 16 are mounted on pedestals 17 and 18 which are made up of insulating material thereby containing the generated heat.
Before commencing operation of the apparatus, the crucible 6 is mounted on the support ring 11, and the initial material is charged into the crucible. A drive is employed to gear 13 to transmit torque to crucible support ring 11 and cylindrical stand 12 to cause rotation of the crucible about its central axis. The drive for rotating vertical rod 3 is energized to begin axial rotation thereof. The side heaters 15 and lower heaters 16 are then started so that the initial material contained in crucible 6 is melted into melt 19. Thereafter, the seed crystal 5 is slowly lowered until it contacts the melt 19. As a result, the seed crystal 5 is partially melted, after which, an equilibrium is established between the seed crystal 5 and the melt 19, so that there is neither melting nor crystallization of the seed crystal 5. The driver is initiated for raising the vertical rod 3 and holder 4 of the crystal seed 5 as well as for the charge to feeding tube 14.
In order to obtain the desired diameter of single crystal 20, the rate at which the vertical rod 3 is raised is held constant and at the same time, the temperature of the bottom heater 16 is controlled. During the stage of growing the single crystal 20 from the seed crystal 4 to the desired diameter, the rate of charge of material from the feed tube 14 is gradually increased in proportion to the mass rate of growth of the single crystal 20. After the desired diameter of the single crystal 20 is achieved, the feed rate from feed tube 14 is then maintained at a constant level, so as a result, the diameter of the single crystal 20 will also remain the same.
In order to obtain the single crystal 20 with the desired properties, it is generally desirable to ensure a stable thermal field at the interface of the single crystal 20 and the melt 19. By coordinating the mass growth rates of the single crystal and feeding of raw material throughout all stages of the growth process, the level of the melt 19 in the crucible 6 will not change its position relative to the side heaters 15 and lower heaters 16. Accordingly, the solid-liquid interface between the single crystal 20 and melt 19 will also not change relative to the side heaters 15 and lower heaters 16. As a result, the thermal conditions will remain stable at this interface of the single crystal 20 and melt 19 to produce a uniform crystal with desired properties.
As shown above, the design of the crucible and its component parts is complex in configuration. Furthermore, the conventional material for the crucible when growing sodium iodide crystals is platinum and the thickness of the crucible wall is about 3 mm. Due to the thickness of the crucible wall, the quantity of platinum required for the crucible is great. As a result, such crucibles are extremely expensive.
Furthermore, the crucible is difficult to reshape into its original configuration after it has been deformed by the temperature variations required by the growth process. As discussed previously, the growth process requires high temperatures for melting the raw material. Additionally, lower temperatures are required for freezing the material to form the crystal. The changes in temperature due to melting and freezing of the material cause deformation of the crucible material. In order to re-use the crucible it must be reshaped into its original configuration for reproducible conditions of the growth process. However, due to its shape and the thickness of the walls, operators have difficulty reshaping the material to its original configuration, after which it can be re-used for growing crystals.
What is needed therefore is a crucible with improved reliability and which can easily be reshaped and re-used for growing crystals after deformation.